The use of UV light—in particular UV-C light—for the purification of water, or more precisely the disinfection and sterilization of water, (hereafter referred, for the sake of simplicity, as water purification or purification of water) is a well-known and well established technical practice. Utilizing UV-C is an efficient way for purifying water reservoirs without the need to add chemicals or for frequent cleaning. UV-C light at sufficiently short wavelengths is mutagenic to bacteria, viruses and other micro-organisms. At a wavelength of around 265 nm, UV breaks molecular bonds of DNA in the cells of micro-organisms, producing thymine dimers in the DNA, thereby destroying the DNA structure necessary to reproduce the cell, rendering them harmless or prohibiting growth and reproduction. The main considerations when designing a UV-C system for UV-C inactivation to be efficient are source intensity and exposure time. Source intensity (also known as lamp intensity) is a function of the sources' radiant energy and the distance to the surface to be irradiated. Once an intensity factor is established it is simple to determine how long the surface needs to be exposed.
In order to have a significant reduction in the micro-organisms present, an energy dosage of over 6000 mJ/cm2 to 8000 mJ/cm2 or greater is required.
More recently, demand has grown for UV-C water purification devices which can utilize technologies from the fast developing field of UV-C LED light sources. It is well known, for example, that semiconductor materials of group IIIA-nitrides (AlxGa1-x-yInyN, [0</=x+y</=1]) have direct band gaps that can be used to generate electromagnetic radiation in the wavelength of ultraviolet (UV). For instance, (AlxGa1-xN (0<x<1)) is often utilized as the component for light emitting diode (LED), generating UV radiation below 365 nm.
In terms of above mentioned water purification, UV-C LED solutions confer numerous advantages over more traditional fluorescent or incandescent UV-C lamps, including for example fast switching capability, small form factor, long lifetime, and a significantly ‘cleaner’ material composition—comprising few hazardous or harmful component materials.
However, UV-C LEDs, as per the state of the art, continue to suffer significant problems with regards efficient light extraction from both LED dies themselves, and also from LED die packages, especially when compared to the advances made in the field of conventional LEDs. The wall plug efficiency of UV-C LEDs is much lower than that of conventional blue LEDs, leading to the effect that the UV-C LEDS generate more heat. In addition, the temperature range in which the UV-C LEDs can be used, so that they provide useful lifetimes that are suitable for practical applications, is much lower than that of blue LEDs. The heating of the junction of the UV-C LED is dramatically reducing the performance of the light source and subsequently is reducing the lifetime. Typical UV-C LED packages or modules use a glass (quartz glass, sapphire or fused silica) window transparent in the UV-C wavelength range, which is attached to a ceramic cavity. The glass window is not directly attached to the die, and emitted light must travel across an intermediary air cavity in order to exit through the glass window. As a result, a significant portion of any generated light is reflected at the window boundary, this reflected light subsequently being lost, since the surrounding cavity typically comprises non-reflective materials such as ceramic (Aluminia, AlN) or metallic Au, Cu.
This poor light extraction efficiency has significant detrimental effects for water purification devices seeking to incorporate UV-C LED technologies. In particular, for the generation of a given required light intensity, or ‘dose’, either a greater density of LED packages is required to be incorporated, or a larger drive current is required to be provided. In the first instance, this will typically incur both greater bulk and weight to the final product, and greater per-unit production costs. In the latter case, running costs are typically increased, and, for a handheld device for example, added weight is also incurred in the form of a larger number of, or greater EMF capacity of, batteries or cells.
Desired therefore is a UV-C LED water purification device, comprising UV-C LED packages having an improved light extraction efficiency, thereby allowing for an increased light intensity capacity, without incurring any increased bulk or weight to the device, nor significantly increasing overall operating costs.